Capacitance-type fluid measuring apparatus



I Feb. 25, I1958 l1R. s. CHILDS v 2,824,449

- cAPAcrrANcE-TYPE FLUIDA MEASURING APPARATUS F11edJu1y29. 1955 NPTY United States Patent Oce tema F CAPACITANCE-TYPE FLUID MEASURING APPARATUS Robert S. Childs, Sudbury, Mass., assigner to Bendix Aviation Corporation, Teterboro, N. J., a corporation of Delaware Application July 29, 1953, Serial No. 371,050

9 Claims. (Cl. 715-304) The present invention relates to measuring and/ or control apparatus and particularly to apparatus for measuring `and/or controlling the level or quantity of fluid' present in a container.

Conventional fluid contents gages for measuring the level or quantity of a luid in a container employ an alternating current bridge having in its measuring arm a measuring condenser adapted for immersion in the fluid to an extent dependent upon the level or quantity of fluid present in the container so that the capacitance of the measairing condenser varies with variation in fluid level to unbalance the bridge to transmit an unbalance signal to indicating means.

In one such known arrangement, `the unbalance signal appearing in the output of the A. C. bridge is simply amplified by a conventional vacuum tube amplifier supplied Afrom a separate D. C. source and the amplified unbalance voltage is applied as one of two separate A. C. input Yvoltages to a special rectifier circuit for a differentially connected direct current measuring instrument.

The A. C. source supplies the second A. C. input voltage. 4When the bridge becomes unbalanced, two direct currents "of variable magnitude are supplied to the measuring in- ;strument, one current being a function of the dilerence of the two A. C. input voltages while the other current is :a function of the sum of the two A. C. input voltages. ln :this known arrangement the measuring instrument serves simply as a null indicator, the comparison or reference Icondenser in the bridge being made adjustable and cali- 'brated in units of fluid quantity so as to rebalance the )bridge by returning the pointer of the measuring instruzment from its left or right hand position to a center-scale ,null condition after each bridge unbalance caused by increase or decrease in fluid level.

It is an object of the present invention to provide new :and improved apparatus for measuring or controlling a jphysical condition or quantity.

Itis another object of the present invention to provide a fluid contents gage of simple rugged construction.

ilt is a further object of the present invention to provide a fluid contents gage which does not employ a null type :indicator but which is sensitive to even small changes in `the quantity of the fluid being measured.

lt is another object of the present invention to provide, vfin a fluid contents gage of the type employing a balance- :able network connected to an indicator, electronic valve .or amplifying means cooperating in a novel manner between the network and indicator to control the indicator lleld in accordance with changes in the fluid quantity being :measured It is a further object of the present invention to provide ra fluid contents gage employing a direct current position indicator as its fluid contents indicating means and which is not only sensitive to small changes in lipid quantity `but is also insensitive to variations in the source voltage @por source frequency.

' Additional objects and novel features of the present invention will become more apparent after consideration of-l the specification and accompanying drawing wherein like' reference numerals indicate similar elements and wherein:v

Fig. l is a schematic diagram of a preferred form ofy ll'uid contents gage in accordance with the present invention;

Fig. 2 is a vector diagram illustrating the directions of the lunidirectional fields developed in the indicating device of Fig. l;

Fig. 3 is a schematic diagram of a somewhat modified form of gage according to the invention;

Fig. 4 is a ve-ctor diagram illustrating the directions of the unidirectional fields developed in the indicating device of Fig. 3;

Fig. 5 is a schematic diagram of another form of gage constructed in accordance with the present invention; and

Fig. 6 is a vector diagram illustrating the directions of the fields developed in the indicating device of Fig. 5.

Referring to Fig. l, there is indicated at 1 a container for a fluid 2. A measuring condenser indicated generally at 3 is suitably mounted in container 1 for vertical immersion of its concentric cylindrical electrodes 4 and S in a dielectric fluid 2 to an extent which depends upon the level of the lluid in the container. The dielectric lluid may be aircraft fuel and the container an aircraft fuel tank. Cylindrical electrode 4 is provided with suitably located apertures 6 so that the level of the lluid between electrodes fl and 5 will always be the same as the level of fluid 'in the rest of the container.

The theory of operation of conventional fluid contents gages depends upon the following considerations. When the container is empty the dielectric between electrodes 4 and 5 will be air which has a dielectric constant value which is unity. When the container is completely filled with a dielectric fluid the dielectric constant between electrodes d and 5 will exceed unity and will have a value depending upon the composition of' the particular fluid present. Since the capacitance of measuring condenser 3 varies directly with the dielectric constant of the dielectric material or materials between its electrodes, it will be appreciated that the capacitance of condenser 3 decreases progressively as the fluid level falls from a full tank condition to an empty tank conditions. Thus, by measuring the capacitance of condenser 3 an indication of fluid level or quantity may be obtained.

The measuring condenser 3 is connected in the measuring arm of a bridge indicated generally at 7. A reference or comparison condenser S is -connected in the reference arm of the bridge. lmpeclances such as resistors 9 and 10 complete the bridge. The bridge input terminals 11 and 12 are connected via conductors 11 and l2 to the secondary winding 13 of a power transformer indicated generally at 14. The primary winding l5 of transformer 14 is connected to a suitable source (not shown) of alternating voltage, whichin the case of an aircraft fuel gage could be the 115 volt, 400 cycle supply line.

If the resistance values of resistors 9 and 10 are equal, then zero calibration of the bridge when the container is empty may be obtained by adjusting the capacitance in the reference arm, for example, by means of the 'trimmer 4condenser ll, until the reference arm capacitance equals the empty capacitance of the measuring condenser 3. When this condition occurs the bridge is balanced so that zero voltage appears between bridge output terminal 16 and the grounded output terminal 17, thereby producing -an empty indication on thescale of the indicating device 18 as will be hereinafter explained in de-tail.

In accordance with the present invention, the bridge output terminal 16 is electrically coupled to the control electrode, such as grid 19 of one section of a'double triode vacuum tube 20, the common cathode or electron emitter assigne 21 for the two sections being connected to ground. The two sections may be separate vacuum tubes instead if so desired. Anodes or collectors 22 and 23 are coupled through respective load resistors 24 and 25 of equal resistance values to a junction point 27 which is connected to one end of a secondary winding 28 of transformer 14, the other end of winding 28 being grounded. Control grids 19 and 29 are biased at the same potential with respect to their common cathode. ln the embodiments illustrated, the control grids are connected to their grounded cathode via respective grid resistors 3f? and 3i of equal resistance values so that the grids are zero biased and when the bridge is balanced the two sections conduct simultaneously to an equal extent on the positive half cycles of the source voltage appearing across secondary winding 28. The pulsating D. C. voltage drops developed across resistors 24 and 25 oppose each other so that under balanced bridge conditions the-re results a net pulsating D. C. voltage of zero magnitude across the series combination of resistors 24 and 25.

The polarities of secondary windings 13 and 28 and the bridge connections are chosen such that with fiuid present in the container to produce bridge unbalance the unbalance potential appearing at output terminal 16 with respect to the grounded output terminal 17 is substantially 180 out of phase with the A. C. potential yapplied to junction point 27 and anodes 22 and 23 with respect to ground. This means that as the magnitude `of the unbalance voltage impressed on grid 19 increases with increasing fiuid level the conduction from cathode 21 to anode 22 on the positive half cycles of the source correspondingly decreases and hence the pulsating D. C. voltage drop across resistor 24 also decreases. Since the potential of grid 29 is not influenced by the unbalance voltage from the bridge, the conduction from cathode 21 to anode 23 and the voltage drop across resistor 25 remain substantially constant during operation of the apparatus over its entire range from empty to full tank conditions. Thus, with fiuid present in the container to produce bridge unbalance there will be developed across the series combination of resistors 24 and 25 a net pulsating D. C. voltage of a magnitude which varies directly with the degree of bridge unbalance. Accordingly, when the tank 1 is completely filled with fuel so that measuring condenser 3 has a maximum capacitance value, the net unidirectional voltage appearing across the series combination of resistors 24 and 25 will have a maximum value.

Alternatively, the polarity of winding 13 or of winding 28 or the bridge connections may be reversed so that the unbalance A. C. voltage applied to grid 19 with respect V to ground may be substantially in phase with the A. C. potential applied to the junction point 27 and to anodes 22 and 23. This would result in increasing rather than decreasing the conduction from cathode 21 to anode 22 during the positive half cycles of the source and would thus have the effect of reversing the polarity of the net unidirectional voltage appearing across the series combination of resistors 24 and 25.

Shown generally at 18 is an indicating device. Device 18 may be a direct current position indicator or a ratiometer which includes a field winding 32 connected in parallel across the series combination of resistors 24 and 25 and a field winding 33 wound approximately 90 apart from winding 32, or in quadrature, so that when windings 32 and 33 lare energized by unidirectional currents they will establish respective unidirectional magnetic fields which are angularly displaced in space approximately 90 from each other, that is in quadrature.

Winding 33 is supplied with a unidirectional current of substantially fixed magnitude by means of a full-wave bridge type rectifier 34 which has its input terminals connected across secondary winding 35 ofk 'transformer 14. It will be appreciated that by virtue of this arrangement any fluctuations in themagnitudeor frequency of the source coupled to theginput of `the 4measuring bridge 7 which would otherwise introduce an error in the reading of indicator 18 will be substantially compensated by a corresponding corrective influence exerted on the field of winding 33 by secondary winding 35 and bridge recti fier 34.

The indicating device 18 has ra rotor which may be a permanent magnet rotor permanently magnetized across a diameter thereof so as to align itself with the axis of the resultant field produced by windings 32 and 33. When rotor 36 rotates it turns a pointer 37 which moves over a scaie calibrated in units of fluid level or fiuid quantity.

As previously described, when tank 1 is empty and bridge 7 is balanced, the net unidirectional voltage appearing across the series combination of resistors 24 and 25 is zero and hence there is no current through Winding 32 to develop a eld at right angles to the constant field established 'oy winding 33. Thus, under empty tank conditions the rotor will line up with the magnetic field of winding 33 and position pointer 37 on the empty mark of scale 38.

As stated previously, when fiuid is present in the tank, a net unidirectional voltage appears across thc series combination of resistors 24 and 25 having a magnitude which is a direct function of the degree of bridge unbalance and hence a direct function of the level or quantity of fiuid present in the container. Hence, a unidirectional current will flow through winding 32 and develop a unidirectional magnetic field, represented by vector ob in Fig. 2, of a magnitude which is a direct function of the level or quantity of fiuid present in the container. As indicated in Fig. 2, the field ob combines with the field 0a established by winding 33 to yield a resultant magnetic field or which is the vector sum of fields 0a and 0b and is displaced from the axis of iield on by an angle A which is proportional to the level or quantity of fluid present iu the tank. The rnagnetized portion of rotor 36 will line up with the axis of the resultant field 0r and position the pointer 37 at the appropriate mark on scale 38 to indicate the quantity of fiuid present in the tank.

The gage illustrated in Fig. 3 is similar to the gage of Fig. l except that the anode load resistors 24 and 25 are omitted and the winding 32 is provided with a center tap which is connected to the ungrounded end of transformer secondary Winding 2S. It will be appreciated that when tank 1 is empty and bridge 7 is balanced the two halves of winding 32 will develop equal but opposed unidirectional fields at right angles to the constant field established by winding 33. Thus, as in the case of the gage of Fig. l, the rotor will line up with the unidirectional field established by winding 33 and the pointer will be positioned at the empty scale mark.

Fig. 4 illustrates vectorially the fields produced in the gage of Fig. 3 when there is fluid present in the tank and the bridge is unbalanced. Vector ob" represents the fixed unidirectional field established in the lower half of winding 32 by the constant current flowing from anode 23, while vector 0b' represents the opposing unidirectional field developed by the current fiowing from cathode 21 to anode 22 and through the upper half of winding 32. The effective field produced by the subtraction of field 0h from field ob is represented by vector 0b. Field ab combines with the field 0a established by winding 33 to produce the resultant field or which is the vector sum of fields on and 0b and is displaced from eld on by an angle A which is proportional to the level or quantity of fluid present in the tank. Rotor 36 will line up with field or and position pointer 37 so as to indicato the level or quantity of fiuid present.

ln the modified gage of Fig. 5 there is employed a direct current position indicator 39 having three field windings 40, 4l and 42 wound approximately 120 apart so that when energized by unidirectional current they will establish, respectively, three unidirectional fields displaced from each other by approximately 120. When the tank is empty and the bridge is balanced unidirectional curalignment with the fixed field established by winding 42.

Thus, under a balanced bridge condition when the tank is empty, the rotor will line up with the resultant unidirectional field which is the sum of the field established byV winding 42 and the field components in alignment with the latter field and produced by windings 40 and 41. The aligned rotor will locate pointer 37 on the empty mark.

Fig. 6 illustrates vectorially the fields produced in the gage of Fig. when there is fiuid in the tank and the bridge is unbalanced. Vector oa represents the fixed unidirectional field established by the constant current flowing through winding 42. Vector ob represents the fixed unidirectional field established in winding 41 by the constant current from anode 23. Field ob has a compo-nent obl in 'alignment with field oa and a component abz at right angles with field oa. Vector ob' represents the unidirectional field developed in winding 40 by the current from anode 22,. Field ob has a component obl' in alignment with field 0a and a component obz' at right angles to field ova. Vector 0b represents the effective field component at right angles'to field oa produced by the subtractionA of field component obj-from field component Ibg. Field ob combines with the sum of the field oa and field components obl' and obl to produce the resultant or total field or which is the vector sum of fields 0a, 0b and ob". Total field or is displaced from field 0a and field components obl and obl by an angle A which is proportional to the level or quantity of fiuid present in the tank. Rotor 36 will line up with field or and position the pointer to indicate the fiuid level or quantity present.

ln those applicatoins where it is desirable to control the magnitude of the condition being sensed, suitable control means 43 of known type may be employed to effect such control. For example, in the embodiments described herein, suitable switch means may be coupled to rotor 36 and be adapted to close when the fuel falls to a predetermined level to energize an electric motor which in turn controls a fuel pump for refilling tank 1 with fuel or connects in a reserve tank with its pump.

it will be appreciated that the apparatus according to the present invention not only is sensitive to small changes in the magnitude of the condition being measured but also is insensitive to fluctuations in the source voltage or source frequency for twol reasons. Because of the balanced arrangement made possible by the provision of the two vacuum tube sections, variations in the voltage applied to anode 22 of the first section will also be experienced by anode 23 of the second section so that variations in differential current become second order. Also, by supplying the bridge input and fixed field winding of the indicator from the same source, variations in the signal field winding means and the fixed field winding means produced by source variations are essentially the same so that no false rotation of the rotor will occur.

It is to be understoodthat the expression dielectric flui includes dielectric liquids and dielectric material in particle, powder or granule form.

lt is also to be understood that the invention is not limited in its application merely to fiuid contents gages or capacitance measuring apparatus but rather may be embodied in apparatus for measuring and/ or controlling other physical conditions or quantities.

While I have illustrated and described certain specific embodiments of my invention by way of example, it will be apparent to those skilled in the art that various changes and modifications may be made in the form of the apparatus disclosed without departing from the spirit of my invention as definedby the appended claims.

What is claimed: y, I,

l. Apparatus for measuring the quantity of a dielectric? fluid present in a container, comprising a measuring condenser having its electrodes adapted for mounting in the container for immersion in the fiuid therein to an extent depending upon the level of the fluid therein so that its capacitance changes with the fluid level, an electrical. bridge having said measuring condenser connected in its measuring arm and having its input adapted to be electrid cally coupled to a source of alternating current to pro duce bridge balance when the container is empty and to develop in its output when there is fluid present in the container an unbalance voltage of a magnitude which is a function of the quantity of fluid present in the container, indicating means having a rotor and first and ,second field; Winding means, means for energizing said first winding` means with unidirectional current to establish a firstfu'ni,- directional field of substantially constant magnitude dur-, ing the operation of the apparatus tending to hold said,v rotor in a first angularposition, first and second amplify. ing devices each having a control electrode, an electron emitterv and a collector, means for electrically coupling the control electrode of said first device to the output of said bridge to receive and amplify said unbalance voltage and for connecting the control electrode of said second device so that said last-mentioned electrode does not receive said unbalance voltage, means for maintaining the control electrodes of said devices at substantially the same potential when said bridge is balanced, means for electrically coupling the emitter and collectors of said devices to the source so that said devices conduct on saidfirst device is changed to an extent depending upon the magnitude of the unbalance voltage when said bridget is unbalanced, and means including said amplifying de-.'

vices and utilizing the difference in the currents fiowing,` therethrough upon bridge unbalance for energizing said second winding means to produce a second unidirectional field of an edective magnitude which is a function of themagnitude of the unbalance voltage and angularly dis,-A placed with respect to said first field to produce a re-i sultant unidirectional field which locates said rotor an angular position corresponding to the quantity of fiui'd present in the container.

2. Apparatus according to claim l wherein the polarities of the bridge connections and of the connections for connecting the emitters and collectors to the source are such that the unbalance voltage supplied to the control electrode of said first device is substantially out of phase with the source voltage supplied to the collectors of said devices, whereby upon bridge unbalance the control electrode of said first device causes the current fiowing through said first device to have a value less than the value of the current flowing through said second device to an extent depending upon the degree of bridge unbalance.

3. Apparatus according to claim l wherein the emittercollector circuits of said devices are adapted to be electrically coupled in parallel circuit relation across the source.

4. Apparatus for measuring the quantity of a di-. electric fiuid present in a container, comprising a measuring condenser having its electrodes adapted for mounting in the container for immersion in the fluid therein to an. extent depending upon the level of the fluid therein so that l its capacitance changes with the fluid level, an electrical bridge having said measuring condenser connected in its measuring arm and having its input adapted to be electrically coupled to a source of alternating current tov produce bridge balance when the container is empty and to develop in its output when there is fiuid present in the container an unbalance voltage of a magnitude which isp;h

assetatiV a function of the quantity of uid present in the containcr, rst and second electron discharge amplifying devices each having a cathode adapted to be electrically coupled to one side of the source and each also having a control electrode and an anode, a first anode load impedance having one terminal connected to the anode of said device, a second anode load impedance having one terminal connected to the anode of said second device and having an impedance value substantially equal to the value of said I'irst load impedance, means for connecting the other terminals of said impedances to a common junction point which point is adapted to be electrically coupled to the other side of the source, means for electrically coupling the control electrode of said iirst device to the output of said bridge to receive and amplify said unbalance voltage and for connecting the control electrode of said second device so thatsaid last-mentioned electrode does not receive said unbalance voltage, means for maintaining the control electrodes of said devices at substantially the same potential when said bridge is balanced so that said devices conduct simultaneously on the same half cycles of the source voltage to an equal extent when said bridge is balanced to produce a net zero voltage drop across the series combination of said impedances and when said bridge is unbalanced the conduction of said second device remains substantially unaltered while the conduction of said first device is altered to an extent depending upon the magnitude of the unbalance voltage to produce a net unidirectional voltage drop across said series combination which is of a magnitude dependent upon the magnitude of the unbalance voltage, and fluid quantity indicating means having a permanent magnet rotor and having rst and second winding means Wound at approximately 90 with respect to each other, means for passing unidirectional current through said first winding means to establish a first unidirectional ield substantially constant in strength during the operation of said apparatus and tending to hold the magnetized portion of said rotor in alignment therewith, said second winding means being connected in parallel across the series combination of said impedances so that when the bridge is balanced no current lows through said second winding means and when the bridge is unbalanced a unidirectional current fiows through said second winding means having a magnitude dependent upon the magnitude of said unbalance voltage to produce a second unidirectional ield displaced in space approximately 90 from said first field and of a magnitude dependent upon the magnitude of the unbalance voltage so that a field is produced which is the resultant of said first and second elds to locate the niagnetized portion of said rotor in an angular position corresponding to the quantity of iiuid present in the container.

5. Apparatus accordingto claim 4 wherein the polaritics of the bridge connections and of the connections for connecting the cathodes and anodes to the source are such that the unbalance voltage supplied to the control electrode of said lirst device is substantially 180 out of phase with the source voltage supplied to the anodes of said devices, whereby upon bridge unbalance the control electrode of said first device causes the current owing through said tirst device and through said rst impedance to have a value less than the value of the current fiowing through said second device and said second impedance to an extent depending upon the degree of bridge unbalance.

6. Apparatus for measuring the quantity of a dielectric iiuid present in `a container, comprising a measuring condenser having its electrodes adapted for mounting in the container for immersion in the iiuid therein to an extent depending upon the level of the fluid therein so that its capacitance changes with the iiuid level, an electrical bridge having said measuring condenser conncctcd in its measuring arm and having its inputadapted to be electrically coupled to a source of alternating current to produce bridge baiance when the container is empty and to develop in its output when there is tiuid present in the container an imbalance voltage of a magnitude which is a function of the quantity of fluid present in the container, first and second electron discharge de vices each` having a cathode adapted to be electrically coupled to one side of the source and each also having a control electrode and an anode, tiuid quantity indicating means having a permanent magnet rotor and having tirst and second field winding means wound at approximately with respect to each other, means for passing a first unidirectional current through said lirst Winding means to establish a first unidirectional iield substantially constant in strength during the operation of said apparatus and tending to hold said rotor in a first angular position, means connecting one end of said second winding means to the anode of said first device and the other end of said second winding means to the anode of said second device, said second winding means being divided into two Winding portions having an equal number of turns with the junction of said two portions adapted to be electrically coupled to the other side of the source, means for electrically coupling the control electrode of said first device to the output of said bridge to receive said unbalance voltage, means for biasing the control electrodes of said devices at substantially the same potential when said bridge is balanced so that said devices conduct simultaneously on the same half cycles of the source to an equal extent when the bridge is balanced thereby producing at substantially right angles to said first field two opposed fields of equal magnitude which nullify each other and when said bridge is unbalanced the current through said first device and its associated winding portion ditiers from the current through said second device and its associated winding portion thereby producing at right angles to said first field two opposed fields of unequal magnitude resulting in an etective unidirectional field which combines with said lirst iield to position said rotor in an angular position correspending to the quantity of fluid present in the con talner.

7. Apparatus according to claim 6 wherein the polarities of the bridge connections and of the connections for connecting the cathodes and anodes `to the source are such that the unbalance voltage supplied to the control electrode of said first device is substantially 180 out of phase with the source of voltage supplied to the anodes of said devices, whereby upon bridge unbalance the control electrode of said first device causes the current .iiowing through said first device and its associated winding portion to have a value less than the value of the current liowing through said second device and its associated winding portion to an extent depending upon the degree ot bridge unbalance.

8. Apparatus for measuring the quantity of a dielectric iiuid present in a container, comprising a measuring condenser having its electrodes adapted for mounting in the container for immersion in the fluid therein to an extent depending upon the level of the fiuid therein so thatits capacitance changes with the fluid level, an electrical bridge having said measuring condenser connected in its measuring arm and having its input adapted to be electrically coupled to a source of alternating current to produce bridge balance when the container is empty and to develop in its output when there is fluid present in the container an unbalance voltage of a magnitude which is a function of the quantity of fluid present inthe container, first and second electron discharge devices each having a cathode adapted to be electrically coupled to one side of the source and each also having a control electrode and an anode, fluid quantity indicating means having a permanent magnet rotor and having first, second and third iield windings mutually displaced approxi* mately with respect to each other, means for passing iirst unidirectional current through said first winding to establish a tirst unidirectional field substantially constant in strength during the operation of said apparatus and tending to hold the magnetized portion of said rotor in a first angular position in alignment with said rst eld, means connecting one end of said second winding to the Ianode of said first device and one end of said third winding to the anode of said second device, said second and third windings having a junction point adapted to be electrically coupled to the other side of the source, means for electrically coupling the control electrode of said first device to the output of said bridge to receive said unbalance voltage, means for biasing the control electrodes of said devices at substantially the same potential when said bridge is balanced so that said devices conduct simultaneously on the same half cycles of the source to an equal extent when the bridge is balanced thereby producing in alignment with said rst iield two aiding eld components and at right Iangles to said rst eld two opposed lield components of equal magnitude which nullify each other and when said bridge is unbalanced the current through said rst device and said second winding differs from the current through said second device and said third winding thereby producing at right angles to said first lield two opposed iield components of unequal magnitude resulting in an effective unidirectional iield which combines with said first eld to position said rotor in an angular position corresponding to the quantity of uid present in the container.

9. Apparatus according to claim 8 wherein the polarities of the bridge connections and of the connections for connecting the cathodes and anodes to the source are such that the unbalance voltage supplied to the control electrode of said lirst device is substantially out of phase with the source voltage supplied to the anodes of said devices, whereby upon bridge unbalance the control electrode of said first device causes the current owing through said first device and through said second winding to have a value less than the value of the current owing through said second device and said third winding to an extent depending Lupon the degree of bridge unbalance.

References Cited in the le of this patent UNITED STATES PATENTS Re. 23,493 Edelman May 20, 1952 2,280,678 Waymouth Apr. 21, 1942 2,338,732 Nosker Jan. 11, 1944 2,344,238 Finch Mar. 14, 1944 2,346,838 Haight Apr. 18, 1944 2,534,802 kSrnith Dec. 19, 1950 2,544,012 Edelman Mar. 6, 1951 OTHER REFERENCES Publication: Strain Gages in Electronics, December 1943, pages 106 to 111 and 192. (Photostatic Copy in 73-88.5.) 

